1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or C-MOS sensor used in a compact image pickup device and more particularly to imaging lenses which are built in image pickup devices mounted in mobile terminals such as smart phones, mobile phones and PDAs (Personal Digital Assistants), which are becoming increasingly compact and thin, and game consoles and information terminals such as PCs.
2. Description of the Related Art
In recent years, the market of mobile terminals including smart phones has been ever-expanding and cameras which cope with an increase in the number of pixels are in the mainstream of cameras built in mobile terminals. Imaging lenses for such cameras are strongly expected to provide higher resolution and be smaller and thinner and also to constitute a high-brightness lens system to cope with an increase in the number of pixels and provide a wide angle of view to capture an image of an object in a wide perspective.
As imaging lenses which meet the trend toward higher performance, many types of imaging lens composed of four lenses (four-element lenses) which can be relatively compact and provide high performance have been proposed.
For example, JP-A-2008-046526 (Patent Document 1) discloses an imaging lens which includes, in order from an object side, an aperture stop, a first lens with positive refractive power, a second lens with negative refractive power, a third lens with positive refractive power, and a fourth lens with negative refractive power having at least one aspheric surface and having a concave object-side surface, in which the power of the first lens and the relation between the curvature radii of the object-side and image-side surfaces of the fourth lens are designed to fail within adequate ranges in order to achieve high performance.
Also, JP-A2008-242180 (Patent Document 2) discloses an imaging lens which includes, in order from an object side, an aperture stop, a first lens with positive refractive power, a second lens with negative refractive power, a third lens with positive refractive power, and a biconcave fourth lens with negative refractive power having at least one aspheric surface, in which the ratios of the focal length of the overall optical system to the focal lengths of the first and third lenses are designed to fall within adequate ranges in order to achieve high performance.
Also, JP-A-2009-014899 (Patent Document 3) discloses an imaging lens which includes, in order from an object side, an aperture stop, a biconvex first lens with positive refractive power, a meniscus second lens with negative refractive power having a convex object-side surface, a meniscus third lens with positive refractive power having a convex image-side surface, and a meniscus fourth lens with negative refractive power having a convex object-side surface, in which the relation between the center thickness of the first lens and the focal length of the first lens and the Abbe numbers of the second and third lenses are designed to fall within adequate ranges in order to achieve high performance.
The imaging lenses described in Patent Document 1 and Patent Document 2 are relatively compact. However, since their F-values are about 3.0, they are not considered to provide sufficient brightness for image sensors which deal with an increasing number of pixels. In addition, the half angle of view is about 30 degrees, which is not enough to meet the demand for wider angles of view. The imaging lens described in Patent Document 3 is relatively compact but its P-value of about 3.2 is not considered to provide sufficient brightness. In addition, its ability to correct spherical aberrations and off-axial aberrations is not sufficient. Therefore, it is difficult for these related art techniques to meet the needs for compactness, wider view angle and small F-value at the same time.